Summary One of the most debilitating features of psychotic disorders is the impairment in social functioning that can begin very early in the illness. Unfortunately, there are no effective treatments available for these impairments. This is in part because their neurobiological basis is not understood. However, recent advances in social neuroscience and neuroimaging have provided novel avenues for understanding the neural circuitry involved in generating social behavior. One general approach often used in the study of human behaviors is to begin by investigating very basic ones first, which may be more amenable than complex processes to reduction to quantifiable relationships between brain responses and behavior. One non-verbal social behavior that can be reliably measured is social spacing or ?personal space? ? the ?comfort zone? or physical distance one maintains from others during social interactions. Studies have found that social spacing is abnormal in patients with schizophrenia. Also, a need to stand physically further away from other people (i.e., a larger personal space) has been repeatedly linked to a desire to avoid social interactions with others, in both healthy people and in patients with schizophrenia. This association suggests that abnormal social spacing could be used as an objective marker of impairments in social motivation. Although the neural mechanisms responsible for social spacing are poorly understood, neurophysiological studies of monkeys and humans have shown that a parietofrontal cortical network, involved in monitoring the space around the body, plays an important role in the social behaviors occurring within this space. Thus, in this proposal, we plan to comprehensively study this neural system, personal space, and social motivation in individuals with a history of psychosis, as well as genetically related and unrelated individuals. In Aim 1 of the project, we will investigate whether variation in the size and/or plasticity of personal space predicts levels of social motivation. In Aim 2, we will examine the function of the parietofrontal network that monitors personal space, to determine whether it plays a specialized role in social behavior and in the defense of the body against social threats. In Aim 3, we will test whether variation in functioning of this parietofrontal system predicts complex types of social perception and experience. Thus, in this project, we will ask whether a basic neural system that monitors the space around the body 1) plays a central role in social behavior and 2) is disrupted in individuals with impaired social motivation. If our hypotheses are confirmed, our results will show that complex social functions, such as those involved in social motivation, can be initiated by basic sensory-motor circuits. Most importantly, we will demonstrate that abnormalities in these neural processes are linked to impairments in social behavior. Lastly, based on these findings, we expect to use this neurobehavioral phenotype in future work as a quantitative marker in studies of novel treatments and genetic investigations.